Innovation in wastewater treatment technology is driven largely by the need to meet increasingly stringent regulatory standards and by the desire to reduce the capital and operating costs of treatment processes. In recent years, these drivers have prompted the emergence of improved biofilm processes such as the Biological Aerated Filter (BAF) and the Moving Bed Biofilm Reactor (MBBR). One of the key advantages of biofilm-based processes is the potentially high volumetric reaction rate that can be attained due the high specific biomass concentration. Unfortunately, this advantage is rarely exploited in full-scale processes as a result of oxygen transfer limitations into thick biofilms. Biofilms in wastewater treatment systems are frequently thicker than the penetration depth of oxygen, typically 50 μm to 150 μm and, under high carbon-loading rates, the process becomes oxygen transfer rate limited. This problem, combined with the difficulty in controlling biofilm thickness has resulted in the application of biofilm technology predominantly for low-rate processes.
Innovative technologies to overcome this problem are mainly based on methods that increase the specific surface area (particle based biofilm technologies), or on methods for increasing the oxidation capacity and efficiency, such as a Membrane-Aerated Biofilm Reactor (MABR). MABR have long been exploited as an innovative wastewater treatment reactor biofilm, comprising a community of microorganisms attached to a surface. Natural immobilization of the microbial community on inert supports allows excellent biomass retention and accumulation without the need for solid-separation devices. In the context of wastewater treatment, the ability of biofilm based processes to completely uncouple solids retention time (SRT) from hydraulic retention time (HRT) is especially useful for slow-growing organisms which would otherwise be washed out of the system, nitrifying biofilms being a case in point. The MABR has several advantages over conventional biofilm technologies;                (1). Comparatively high volumetric carbon oxygen demand (COD) removal rates are achievable if pure oxygen is fully exploited and if biofilm thickness-control measures are in place.        (2). Bubbleless aeration offers the potential for significantly higher oxygen utilization efficiencies with consequent energy savings. In addition, reduced air stripping during the biotreatment of volatile organic compounds is possible.        (3). Simultaneous nitrification, denitrification and COD removal can be achieved at comparatively higher rates due to the unique microbial population stratification.        (4). Specialist degrading microorganisms, such as ammonia oxidizing bacteria, tend to be preferentially located adjacent to the biofilm-membrane interface thereby enhancing their retention by protection from biofilm erosion.        
However, commercial exploitation of the technology has not yet emerged due to the disadvantages as discussed above, and until the present time, there have been very limited trials of the technology beyond laboratory scale. Additional mixing in an aerobic wastewater treatment reactor is generally not required as the bubble aerators or the surface aerators have associated mixing. In anoxic or anaerobic reactors external mixing is required. This is generally carried out by large submerged mixers or jet mixers where the aim is to keep suspended biomass or suspended biomass carriers in suspension and moving throughout the housing. The disadvantage of using bubble or surface aerators and submerged or jet mixers is that they provide effective mixing only in tanks without any restrictions to fluid flow and they have a relatively large energy demand.
Chinese Patent Document CN102531153A describes a MABR having an external conduit for conveying the waste liquid from an upper headspace to a lower headspace. However, the MABR uses a pump situated at the bottom of the housing, which prevents biomass from settling and keeps biomass suspended and moving throughout the housing. S. J You and W. Y. Chen (International Biodeterioration & Biodegradation, vol. 62. pp. 2-249 (2008)) describe a sequencing batch membrane bioreactor using an impellor to mix and maintain the biomass from the liquid waste in suspension, thus making it difficult to remove.
It is an object of the present invention to overcome at least one of the above-referenced problems.